The present invention relates to an angular velocity and acceleration sensor for sensing at least one of angular velocity and acceleration.
Gyroscopes are employed as angular velocity and acceleration sensors for sensing angular velocity and acceleration which are necessary when obtaining information such as the location, direction, position, and velocity of a moving vehicle. Gyroscopes of the vibration type have attracted attention from the viewpoint of low cost and high reliability and one of these vibration-type gyroscopes uses a tuning-fork oscillator.
FIG. 11 is a perspective view showing a portion of an angular velocity and acceleration sensor (disclosed in Japanese patent Laid-Open No. 47913/1985) which uses a conventional tuning-fork oscillator. In this figure, reference number 61 denotes a base, reference numbers 62, 63 vibrating reeds fixed to the base 61, reference numbers 64, 65 mounting-plates mounted on the upper portions of the vibrating reeds 62, 63, and reference numbers 66, 67 vibrating reeds the lower portions of which are mounted on the mounting-plates 64, 65. Each of the vibrating reeds 62, 63, 66, 67 is formed such that the vibrating reeds 62, 63 can vibrate on the main driven vibrating axis, that is to say, the x-axis, and the vibrating reeds 66, 67 can vibrate in the direction which crosses the main driven vibrating axis at right angles, that is to say, the y-axis, and such that the resonance frequency of the vibrating reeds 62, 63 is equal to that of the vibrating reeds 66, 67. By enabling resonance of each of the vibrating reeds, it becomes possible to obtain a large output displacement with a small input.
In this angular velocity and acceleration sensor, when an angular velocity .OMEGA. is produced around the axis z which crosses the xy plane at right angles in the state of the vibrating reeds 62, 63 being vibrated at a resonance frequency .omega. in opposite directions on the x-axis, Coriolis force F which is proportional to the angular velocity .OMEGA. acts on the vibrating reeds 66, 67. In this case, when the amplitude of the vibration on the x-axis is shown by a and time is shown by t, the location x of the vibrating reeds 66, 67 is expressed by the following equation: EQU x=a sin.omega.t (1)
Therefore, the relative velocity x of the vibrating reeds 66, 67 on the x-axis relative to the base 61 is expressed by the following equation: EQU X=a.omega.cos.omega.t (2)
Thus, if the mass of the vibrating reeds 66, 67 is shown by m, Coriolis force F is expressed by the following equation: EQU F=2m.OMEGA.x=.OMEGA.2ma.omega.cos.omega.t (3)
As seen from the equation (3), when the Coriolis force F acts, the vibrating reeds 66, 67 are vibrated in opposite directions to each other on the y-axis. Since the amplitude is proportional to the angular velocity .OMEGA., the angular velocity .OMEGA. can be determined by sensing the amplitude of the vibrating reeds 66, 67. The amplitude can be sensed as an electrical output which is proportional to the transformed displacement of piezoelectric materials stuck to the flat elements of the vibrating reeds 66, 67. When an acceleration y is produced on the y-axis, the vibrating reeds 66, 67 are transformed in proportion to the acceleration y in the same direction on the y-axis and the acceleration y can be sensed by sensing the transformed displacement of the vibrating reeds 66, 67. This transformed displacement can be obtained as the output from the same piezoelectric material as in the amplitude. The angular velocity can be obtained by summing the outputs of the vibrating reeds and the acceleration can be obtained by subtracting the output of one vibrating reed from that of the other.
As described above, in a vibration-type gyroscope, it is necessary to equalize the resonance frequency of the vibrating reeds on the x-axis to that on the y-axis, as well as the resonance frequencies of all the vibrating reeds in a group. A resonance frequency f of vibrating reeds is expressed as follow: ##EQU1## an error rate is expressed as follows: ##EQU2## and the error rate of the resonance frequency becomes 3 times the dimensional error rate of a vibrating reed, wherein k denotes the spring constant of a vibrating reed, m the mass, and b, h, and l, the width, the thickness, and the length of a vibrating reed, respectively. The equation (4) is established with the proviso that there is no error in the mass of the mounting-plates 64, 65. An error rate in terms of resonance frequency of 1% or less is required and the smaller the vibrating reeds, that is, the smaller the absolute value of l and h, the smaller the value of the permissible error.
Therefore, in such a conventional angular velocity and acceleration sensor, since the vibrating reeds 62, 63, and the vibrating reeds 66, 67 are mounted on the base 61 with the mounting-plates 64, 65 interposed therebetween, this sensor has a complicated structure, is difficult to work and assemble, and easily produces errors during assembly, resulting in poor detection accuracy and difficulties in achieving miniaturization.